In this short introduction to SIP, we’ll not cover all SIP message types and headers. Instead, we’ll take a brief look at the SIP INVITE message, along with a few common header lines. Let us again suppose that Alice wants to initiate a VoIP call to Bob, and this time Alice knows only Bob’s SIP address, bob@domain.com, and does not know the IP address of the device that Bob is currently using. Then her message might look something like this: